1. Field of the Invention
This invention relates to methods for the production of a patterned calcined inorganic film and a plasma display panel. More particularly, this invention relates to a technique for calcining a patterned film without inducing warpage, shrinkage of line width, etc. in the formation of a circuit on a ceramic substrate, the manufacture of a photoelectric tube, the manufacture of an electroconductive film or an insulating film on the front substrate or the back substrate of a plasma display panel, and the formation of a fluorescent film.
2. Description of the Prior Art
In recent years, the demand for conductor circuit patterns of increased density and heightened fineness has been escalating in the field of circuit substrates, displays, and the like. As a result, the desirability of developing a patterning technique which is capable of forming highly fine patterns with high reliability has been finding recognition.
When a patterned calcined inorganic film is produced by using a composition containing a heat decomposable binder and particles of an inorganic material, forming on a substrate a patterned film of the composition by the printing technique or photolithographic technique, and calcining this film thereby burning off the heat decomposable binder, there arises a peculiar problem which is attendant on the operation of calcination. Now, this problem will be explained below with reference to a plasma display panel (hereinafter abbreviated as "PDP"), for example.
The PDP is a planar display for exhibiting pictures and pieces of information by utilizing the light emitted by plasma discharge. It is classified under the DC type and the AC type according to the structure of panel and the mode of driving. The principle of color display by the PDP consists in generating plasma discharge in cells (discharge spaces) between two opposed electrodes severally formed on a front glass substrate and a back glass substrate separated by intervening ribs (barriers), and exciting the phosphor formed on the inner surface of the back glass substrate with the ultraviolet light generated by the discharge of such a gas as He or Xe sealed in the cells thereby inducing generation of visible lights of three primary colors. The cells in the DC type PDP are divided by the component ribs of a lattice, whereas those in the AC type PDP are divided by the ribs which are parallelly arranged on the face of the substrate. In either case, the cells are divided by ribs.
FIG. 1 illustrates a typical construction of the planar discharge type PDP using a three-electrode structure for full color display. On the lower face of a front glass substrate 1, many pairs of display electrodes 2a, 2b each comprising a transparent electrode 3a or 3b intended for discharge and a bus electrode 4a or 4b intended for lowering the line resistance of the transparent electrode are formed. On the display electrodes 2a, 2b, a transparent dielectric layer 5 (low melting glass) for accumulating electric charge is formed by printing and calcination. A protective layer (MgO) 6 is formed thereon by vacuum deposition. The protective layer 6 assumes the role of protecting the display electrodes and maintaining the state of discharge.
On a back glass substrate 7, ribs (barriers) 8 shaped like stripes and adapted to partition discharge spaces and address electrodes (data electrodes) 9 severally disposed in the discharge spaces are formed with prescribed pitches. On the inner faces of discharge spaces, fluorescent films of the three colors, i.e. red (10a), blue (10b), and green (10c), are laid out regularly. In the full color display, the fluorescent films of the three primary colors of red, blue, and green mentioned above jointly form one picture element.
The PDP described above is called a "planar discharge system" because an AC pulse voltage is applied between the pair of display electrodes 2a, 2b to induce discharge between the electrodes on one and the same substrate. It has a construction such that the ultraviolet light generated by discharge excites the fluorescent films 10a, 10b, and 10c of the back substrate 7 and the visible light consequently generated is seen through the transparent electrodes 3a, 3b of the front substrate 1 (reflection type).
The formation of the bus electrodes 4a, 4b mentioned above has been heretofore attained by a process which comprises forming three Cr/Cu/Cr layers by vacuum deposition or sputtering on the transparent electrode and then patterning the layers by the photolithographic technique. Since this process suffers copiousness of component steps and high cost of production, in recent years, a process which comprises screen printing a conductive paste such as of silver and then calcining the layer of the paste or, for the purpose of imparting to a pattern a line width of not more than 150 .mu.m, a process which comprises applying a photosensitive conductive paste to the transparent electrode, exposing the layer of the paste to light through a pattern mask, developing the exposed layer, and subsequently calcining the developed layer has come to take the place of the former process.
The calcining step, however, has encountered the problem of exposing the bus electrodes 4a, 4b to warpage, as shown in FIG. 2, to shrinkage of line width, and to accidental breakage. Not only when the breakage of electrode lines occurs but also when the shrinkage of line width occurs, the problem arises that the PDP ceases to operate normally because the bus electrodes are no longer capable of accomplishing the primary object thereof to lower the line resistance of the transparent electrodes 3a, 3b. When the bus electrodes 4a, 4b develops warpage, the problem ensues that the bus electrodes become liable to cause a short circuit because the dielectric layer on the warped parts of the bus electrodes suffers a decrease in thickness.
The problem of entailing the warpage, shrinkage of line width, and breakage of electrode lines subsequent to the calcination mentioned above occurs not only in the bus electrodes 4a, 4b but also in the address electrodes 9.
The address electrodes 9 are manufactured by forming a patterned layer of an electroconductive paste containing an electroconductive power of Ag, Au, Pd, Ni, Cu, Al, Pt, or the like on the back glass substrate 7 and calcining the layer at a temperature in the approximate range of 500.degree. to 600.degree. C. For the formation of the pattern of the electroconductive paste, the printing method, the lift-off method (the method which comprises laminating a photosensitive dry film onto a substrate throughout the whole surface thereof, patterning the film on the substrate, filling up grooves opened in the film with an electroconductive paste, drying and hardening the paste, then removing the dry film, and calcining the hardened paste; otherwise called "dry film filling method"), and the method of patterning a photosensitive electroconductive paste by photolithography (the method which comprises applying a photosensitive electroconductive paste to a substrate, drying the applied layer of the paste, exposing the dry layer to light through a pattern mask, developing the exposed areas, and calcining the developed layer) are adopted. The address electrodes likewise suffer the problem of encountering such difficulties as warpage, shrinkage of line width and breakage of electrode lines during the course of calcination and preventing the PDP from normally operating.
The problems to be encountered in the manufacture of a conductor pattern as described above are not limited to the PDP. They also occur in various other operations such as, for example, forming a circuit by forming a patterned circuit of an electrodoncuctive paste or photosensitive conductive paste on a ceramic substrate and calcining the patterned circuit and forming a conductor pattern through a calcining step as in the manufacture of a photoelectric tube.
They occur not only in the formation of a conductor pattern but also in the formation of a nonconductor (insulator) pattern. In the PDP mentioned above, for example, the formation of ribs (barriers) is effected by the printing method which comprises performing the work of printing and drying a glass paste by screen printing on a glass substrate up to eight-ten-odds repetitions till the superposed layers in a prescribed pattern amount to a prescribed thickness (about 100-150 .mu.m) and then calcining the superposed layers of glass paste; the sandblasting method which comprises applying glass paste to a glass substrate throughout the entire surface till a prescribed thickness, scraping out grooves in the layer of glass paste to a prescribed pattern by blowing blast powder thereto through a pattern mask formed by photolithography and vested with an ability to resist blasting, and then calcining the patterned layer; and the lift-off method (dry film filling method) which comprises laminating a photosensitive dry film of a prescribed thickness onto a glass substrate, forming grooves in the film by photolithography in the prescribed shape of ribs, applying glass paste so as to fill up the grooves, drying and hardening the glass paste, removing the photosensitive dry film from the substrate, and calcining the glass paste.
The ribs formed on the PDP back substrate are intended to confine the luminous discharge within a fixed area so as to preclude false discharge or cross talk between adjacent discharge cells and ensure ideal display. They are endowed with the function of retaining uniform discharge spaces owing to their own height, width, and pattern gap and enhancing the mechanical strength of the whole panel. In order for the PDP to acquire high luminance, it is necessary that the discharge gas spaces be as wide as permissible and the ribs be as thin as possible. Specifically, it is necessary to form ribs which have a large aspect ratio (ratio of height to width), a narrow width, a great height, and fully sufficient strength.
When the warpage or the shrinkage of line width occurs during the course of calcining ribs mentioned above, the ribs acquire only disfigured skirts and inaccurate heights and the display cells suffer their shape to be heavily affected by blurred borders of the ribs and the panel eventually obtained displays pictures of inferior quality.
When the warpage or the shrinkage occurs in the formation of a fluorescent film, it possibly cause a crack or peeling of the film and impairs the quality of display and degrades the yield.